Recently, the number of automobiles is exponentially increasing, and an amount of exhaust gas discharged from an internal engine of the automobile rapidly goes on increasing. Particularly, various substances including in the exhaust gas discharged from a diesel engine are a cause raising air pollution and give a serious influence upon natural environment. Also, there are lately reported study results that particulates in the exhaust gas (diesel particulate) are a cause raising an allergic obstacle and a reduction of spermatozoon number, so that it is an urgent issue for humans to take a countermeasure for removing the particulates in the exhaust gas.
Under such a situation, there have hitherto been proposed various apparatuses for the purification of the exhaust gas. As the general apparatus for the purification of the exhaust gas, there is, for example, a structure that a casing is arranged on the way of an exhaust pipe connected to an exhaust manifold of an engine and a filter having fine holes is arranged therein. As a material constituting the filter, there are metals, alloys and ceramics. As a typical example of the filter made of the ceramic is well-known a cordierite filter. Recently, porous silicon carbide sintered body having such merits that the heat resistance, mechanical strength and catching efficiency are high, and the chemical stability is good, and the pressure loss is low and the like is used as an example of a filter-forming material.
The general ceramic filter and the exhaust gas purification apparatus using the same are such constructed that when the exhaust gas passes through many cells formed along an axial direction of the filter, fine particles in the exhaust gas are trapped in the cell walls to remove the fine particles from the exhaust gas. However, a ceramic filter made of a porous silicon carbide sintered body is generally large in the thermal expansion and weak to thermal shock, so that there is a problem that as the size becomes larger, crack is apt to be easily produced. In this connection, JP-A-8-28246 discloses a ceramic filter being an aggregate formed by combining plural small filter units as a countermeasure for avoiding the breakage due to cracks. This technique is a method wherein a honeycomb shaped body of a square pole (honeycomb unit) is formed by continuously extruding a ceramic raw material through a die of an extrusion shaping machine and cut into an equal length to form cut pieces, and then the cut pieces are sintered to form filter units, and a plurality of such filter units are integrally bundled while adhering outer peripheral faces with each other through a ceramic adhesive to form an aggregate of these filter units as a ceramic filter.
Moreover, the ceramic filter is preferable to be wound on its outer peripheral face with a matt-shaped heat insulating material such as ceramic fibers or the like, and is received in a casing disposed on the way of an exhaust pipe of an automobile or the like at such a state.
In such a filter, as the catching and burning (regeneration) of soot are repeated, there may be caused the scattering of the catching amount of soot in accordance with the position of the ceramic filter.
On the other hand, JP-A-1-145377 proposes a filter of a honeycomb structure in which an average pore size is stepwise or continuously increased from a partition wall of a central portion toward a partition wall of an outer peripheral portion.
The above conventional technique (JP-A-1-145377) aims at only the central portion and the outer peripheral portion. In the ceramic filter having such a structure, when it is arranged in the casing of the exhaust pipe, there may not be carried out the uniform catching and regeneration. That is, a large amount of soot is locally and non-uniformly caught on a part of the filter units and hence the strength as the filter may be lowered in the regeneration or the like. Also, ash content (ash) included in a fuel additive, engine oil or the like is apt to be easily stored on a part of the filter units and hence the service life as the filter may be shortened.
Also, when the aforementioned conventional ceramic filter is used by repeatedly conducting the regeneration over a constant period as a whole, as seen from a graph of a regenerating ratio (a ratio of changing weight before and after the regeneration) shown in FIG. 13, it is confirmed that the soot is surely burnt at an initial stage, but the reactivity as a catalyst is gradually lowered as the regeneration is repeated several times.
In the light of the above problems included in the conventional techniques, it is an object of the invention to provide a ceramic filter in which soot can be equally caught in axial and radial directions of the filter as a whole and it is effective to uniformly conduct the regeneration and the high efficiency of removing the exhaust gas (filter efficiency) can be maintained over a long time and further the durability is excellent.
It is another object of the invention to provide an apparatus for the purification of the exhaust gas using the above ceramic filter having a high filter efficiency and excellent strength and service life.